Aluminum magnesium titanate is a homogeneous solid solution formed over the entire compositional range of aluminum titanate and magnesium titanate. A sintered product of aluminum magnesium titanate has high heat resistance, a small thermal expansion coefficient, and excellent corrosion resistance. The melting point of aluminum magnesium titanate is about 1,640° C. e.g. in the case of a sintered product wherein aluminum titanate and magnesium titanate are solid-solubilized in an approximately equivalent molar ratio. This temperature is lower when compared with the melting point of aluminum titanate of 1,870° C., but it is higher when compared with the melting point of magnesium titanate of 1,600° C. Therefore, in general, the upper limit of heat resistance of aluminum magnesium titanate is inferior to that of aluminum titanate, but it is still superior in comparison with that of magnesium titanate.
On the other hand, an aluminum titanate sintered product and a magnesium titanate sintered product are, respectively, constructed from crystal grains having the pseudobrookite type crystal structure, and their coefficients of thermal expansion are anisotropic. Therefore, when heating and cooling are carried out, slippage by a thermal stress is likely to take place at the crystalline interface, and there is a drawback such that the mechanical strength tends to deteriorate due to progress in micro cracks and apertures. Since aluminum magnesium titanate also has the same drawback, the aluminum magnesium titanate sintered product is also insufficient in mechanical strength. Specifically, in a case where the aluminum magnesium titanate sintered product is used in an application in which high temperatures and heavy loads are applied, it cannot exhibit sufficient durability.
Further, the lower limit of the temperature for synthesizing aluminum titanate is 1,280° C., while the lower limit of the temperature for synthesizing magnesium titanate is 900° C. Both are unstable in the temperature range lower than the temperatures for their syntheses. As a solid solution of these compounds, aluminum magnesium titanate is also unstable at a temperature lower than the temperature range for its synthesis like aluminum titanate and magnesium titanate. Accordingly, if aluminum magnesium titanate is used in a decomposing temperature range for long time, it is likely to undergo thermal decomposition to form TiO2 (rutile) and MgAl2O4 (spinel). Here, the decomposing temperature range for aluminum magnesium titanate varies depending on a solid solution ratio of aluminum titanate and magnesium titanate, but it is usually from about 800 to 1,280° C. For example, in the case of an aluminum magnesium titanate sintered product, in which aluminum titanate and magnesium titanate are solid-solubilized in an approximately equivalent molar ratio, the aluminum magnesium titanate sintered product thermally decomposes into TiO2 (rutile) and MgAl2O4 (spinel) (Non-Patent Document 1) if it is continuously used at a temperature in the vicinity of 1,100° C. for long time.
Therefore, a conventional aluminum magnesium titanate sintered product could not be used continuously in such a decomposing temperature range, and regardless of its high heat resistance, extremely small thermal expansion coefficient and excellent corrosion resistance, the use of the conventional aluminum magnesium titanate was limited, because it had low reliability due to problems in the thermal decomposition property and mechanical strength.    Non-Patent Document 1: Journal of American Ceramic Society, 1998, 81 [10], pp. 2645-2653